Power storage element, manufacturing method thereof, and power storage device

ABSTRACT

Disclosed is a power storage element including a positive electrode current collector layer and a negative electrode current collector layer which are arranged on the same plane and can be formed through a simple process. The power storage element further includes a positive electrode active material layer on the positive electrode current collector layer; a negative electrode active material layer on the negative electrode current collector layer; and a solid electrolyte layer in contact with at least the positive electrode active material layer and the negative electrode active material layer. The positive electrode active material layer and the negative electrode active material layer are formed by oxidation treatment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/797,104, filed Mar. 12, 2013, now allowed, which claims the benefitof foreign a priority application filed in Japan as Serial No.2012-069536 on Mar. 26, 2012, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power storage element, amanufacturing method of the power storage element, and a power storagedevice.

A power storage element in this specification refers to any elementwhich has a function of storing power, and a power storage device inthis specification refers to any device which has a function of storingpower and in which a plurality of power storage elements are arranged ina plane.

2. Description of the Related Art

In recent years, a variety of power storage devices such as lithiumsecondary batteries, lithium-ion capacitors, and air cells have beendeveloped. In particular, a lithium secondary battery in which chargeand discharge are performed by transfer of lithium ions between apositive electrode and a negative electrode has been attractingattention as a secondary battery with high output and high energydensity.

A lithium secondary battery refers to a secondary battery where lithiumions are used as carrier ions. Examples of carrier ions which can beused instead of lithium ions include alkali-metal ions such as sodiumions and potassium ions; alkaline-earth metal ions such as calcium ions,strontium ions, and barium ions; beryllium ions; and magnesium ions.

Many of conventional electrolytes of lithium secondary batteries areliquid electrolytes which have high lithium conductivity at roomtemperature, and organic electrolytic solutions are used in many ofcommercially available lithium secondary batteries. However, lithiumsecondary batteries which contain organic electrolytic solutions haverisks of leaking the electrolytic solutions and catching fire;therefore, an all-solid-state battery which contains a solid electrolyteand has a high level of safety has been actively researched (see PatentDocument 1).

Further, a secondary battery where a positive electrode and a negativeelectrode are formed over one of surfaces of a substrate and a solidelectrolyte is provided between the positive electrode and the negativeelectrode has been developed (see Patent Document 2).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123081-   [Patent Document 2] Japanese Published Patent Application No.    2006-147210

SUMMARY OF THE INVENTION

In the above secondary battery where a positive electrode and a negativeelectrode are formed over one of surfaces of a substrate and a solidelectrolyte is provided between the positive electrode and the negativeelectrode, a physical short circuit between the positive electrode andthe negative electrode is minimized and a separator is unnecessary. Forthis reason, the battery can be manufactured at low cost. However,layers (e.g., a positive electrode current collector layer, a positiveelectrode active material layer, a negative electrode current collectorlayer, a negative electrode active material layer, and a solidelectrolyte layer) included in the battery are stacked in order; thus,the number of steps is large, leading to a long manufacturing process.Accordingly, the manufacturing cost increases.

In view of the above, an object of one embodiment of the presentinvention is to provide a power storage element which includes apositive electrode and a negative electrode provided so as to be levelwith each other (that is, arranged in the same plane) and which can beformed through a simple process, a formation method of the power storageelement, and a power storage device where a plurality of the powerstorage elements are arranged so as to be level with each other.

One embodiment of the present invention is a power storage elementincluding a positive electrode current collector layer and a negativeelectrode current collector layer which are level with each other; apositive electrode active material layer on the positive electrodecurrent collector layer; a negative electrode active material layer onthe negative electrode current collector layer; and a solid electrolytelayer in contact with at least the positive electrode active materiallayer and the negative electrode active material layer. The positiveelectrode active material layer contains a metal oxide containing ametal element which is contained in the positive electrode currentcollector layer. The negative electrode active material layer contains ametal oxide containing a metal element which is contained in thenegative electrode current collector layer.

Further, the power storage element may include a lithium layer whichoverlaps with at least one of the positive electrode active materiallayer and the negative electrode active material layer.

One embodiment of the present invention is a power storage device wherea plurality of the power storage elements are arranged so as to be levelwith each other. The plurality of power storage elements areelectrically connected to each other through a wiring, thereby beingconnected in series or in parallel in the power storage device.

One embodiment of the present invention may be provided with a means forswitching between series connection and parallel connection of the powerstorage elements. As the switching means, for example, a transistor maybe used.

One embodiment of the present invention is an electrical deviceincluding the above power storage element and the above power storagedevice.

One embodiment of the present invention is a manufacturing method of apower storage element. The method includes the steps of forming apositive electrode current collector layer and a negative electrodecurrent collector layer so that they are level with each other;performing oxidation treatment on the positive electrode currentcollector layer and the negative electrode current collector layer toform a positive electrode active material layer on a surface of thepositive electrode current collector layer and a negative electrodeactive material layer on a surface of the negative electrode currentcollector layer; forming a solid electrolyte layer in contact with atleast the positive electrode active material layer and the negativeelectrode active material layer; and forming a lithium layer overlappingwith at least one of the positive electrode active material layer andthe negative electrode active material layer. The positive electrodeactive material layer contains a metal oxide containing a metal elementwhich is contained in the positive electrode current collector layer.The negative electrode active material layer contains a metal oxidecontaining a metal element which is contained in the negative electrodecurrent collector layer.

The oxidation treatment for the positive electrode current collectorlayer and the negative electrode current collector layer can beperformed by oxygen plasma treatment. For an oxygen atmosphere of theoxygen plasma treatment, an oxygen gas, a dinitrogen monoxide gas, anozone gas, or the like may be used. An oxidation treatment apparatus isnot particularly limited as long as it is provided with a plasmageneration mechanism; for example, a plasma CVD apparatus or asputtering apparatus can be used.

Alternatively, anodic oxidation treatment may be performed for oxidationtreatment. Still alternatively, heat treatment in an oxygen atmospheremay be performed. For the oxygen atmosphere, any of the above gaseswhich can be used for the oxygen plasma treatment can be used.

As the positive electrode active material layer, a layer which containsvanadium oxide or manganese oxide may be used.

As the negative electrode active material layer, a layer which containsniobium oxide, copper oxide, cobalt oxide, nickel oxide, iron oxide,tungsten oxide, molybdenum oxide, or tantalum oxide may be used.

In one embodiment of the present invention, as the solid electrolytelayer in the power storage element or the power storage device, a layerwhich contains a compound containing lithium and sulfur, a compoundcontaining lithium and oxygen, or lithium oxide may be used.

Thus, according to the present invention, the oxygen treatment performedon the positive electrode current collector layer and the negativeelectrode current collector layer enables formation of the positiveelectrode active material layer and the negative electrode activematerial layer on the surface of the positive electrode currentcollector layer and the surface of the negative electrode currentcollector layer, respectively. Accordingly, the manufacturing processcan be simplified, leading to a reduction in manufacturing cost.

According to one embodiment of the present invention, a power storageelement including a positive electrode and a negative electrode providedso as to be level with each other and which can be formed through asimple process, a manufacturing method of the power storage element, anda power storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are top views and a cross-sectional view which illustratea power storage device of one embodiment of the present invention;

FIGS. 2A to 2D are cross-sectional views which illustrate a formationmethod of a power storage element of one embodiment of the presentinvention;

FIGS. 3A to 3C are top views and a cross-sectional view which illustratea power storage device of one embodiment of the present invention;

FIGS. 4A to 4D are cross-sectional views which illustrate a formationmethod of a power storage element of one embodiment of the presentinvention;

FIG. 5A to 5D are cross-sectional views which illustrate the formationmethod of a power storage element of one embodiment of the presentinvention;

FIG. 6 is a cross-sectional view which illustrates a power storageelement of one embodiment of the present invention;

FIGS. 7A and 7B are a block diagram and a circuit diagram whichillustrate a power storage device of one embodiment of the presentinvention;

FIGS. 8A and 8B are circuit diagrams which illustrate a power storageelement array of one embodiment of the present invention;

FIG. 9 illustrates electrical devices; and

FIG. 10A to 10C illustrate an electrical device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described with reference tothe drawings. Note that the present invention is not limited to thefollowing description and it will be readily appreciated by thoseskilled in the art that modes and details can be modified in variousways without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the embodiments. In descriptionusing the drawings for reference, in some cases, common referencenumerals are used for the same portions in different drawings. Further,in some cases, the same hatching patterns are applied to similarportions, and the similar portions are not necessarily designated byreference numerals.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

(Embodiment 1)

In this embodiment, a power storage element, a formation method thereof,and a power storage device of one embodiment of the present inventionwill be described with reference to FIGS. 1A to 1C and FIGS. 2A to 2D.

FIGS. 1A to 1C illustrate a power storage element and a power storagedevice in this embodiment. FIG. 1A is a top view of a power storagedevice 900 in this embodiment which shows the case where power storageelements 200 are arranged so as to be level with each other. FIG. 1B isa top view of the power storage element 200, and FIG. 1C is across-sectional view along dashed dotted line A-B in FIG. 1B. Note thatin FIG. 1B, a sealing layer 114 and the like are omitted for simplicity.

Further, in this embodiment, a power storage element will be describedin which a vanadium layer is used as a positive electrode currentcollector layer; a vanadium oxide layer is used as a positive electrodeactive material layer; a niobium layer is used as a negative electrodecurrent collector layer; and a niobium oxide layer is used as a negativeelectrode active material layer.

Note that for convenience, a power storage device in this specificationrefers to the structure where a plurality of power storage elements arearranged so as to be level with each other. Although FIG. 1A illustratesthe case where the power storage elements 200 are arranged in a matrix,one embodiment of the present invention is not limited thereto. Randomarrangement may be employed and the positions of opposing positive andnegative electrodes may be changed as appropriate. Although notillustrated, a wiring, a terminal, or the like connected to each powerstorage element 200 may be provided.

The power storage element 200 in FIG. 1C includes a substrate 100, aninsulating layer 102 over the substrate 100, a vanadium layer 104 and aniobium layer 106 which are level with each other and are over theinsulating layer 102, a vanadium oxide layer 108 over the vanadium layer104, a niobium oxide layer 110 over the niobium layer 106, and a solidelectrolyte layer 112 in contact with at least the vanadium oxide layer108 and the niobium oxide layer 110. The vanadium layer 104 and thevanadium oxide layer 108 function as a positive electrode, and theniobium layer 106 and the niobium oxide layer 110 function as a negativeelectrode. Further, a sealing layer 114 is preferably formed over thevanadium oxide layer 108, the niobium oxide layer 110, the solidelectrolyte layer 112, and the insulating layer 102.

In FIG. 1C, a lithium layer 116 which overlaps with the niobium oxidelayer 110 is formed over the solid electrolyte layer 112. This lithiumlayer is for supplying lithium serving as a carrier to the niobium oxidelayer 110 serving as a negative electrode active material in the powerstorage element 200 (for predoping the niobium oxide layer 110 withlithium). Although predoping the niobium oxide layer 110 with lithium isdescribed above, the vanadium oxide layer 108 may alternatively bepredoped with lithium. The lithium layer 116 is formed so as to overlapwith at least one of the vanadium oxide layer 108 and the niobium oxidelayer 110 and may be formed over the entire surface over which thelithium layer 116 is to be formed. Further, a copper layer or a nickellayer may be formed in contact with the lithium layer 116 (notparticularly illustrated). The copper layer or the nickel layer has ashape substantially the same as that of the lithium layer 116. Thecopper layer or the nickel layer can function as a current collectorwhen the niobium oxide layer 110 is predoped with lithium from thelithium layer 116.

The lithium layer 116 is formed so as to overlap with the niobium oxidelayer 110 in this manner and discharging is performed with the niobiumlayer 106 and the niobium oxide layer 110 serving as a positiveelectrode and the lithium layer 116 serving as a negative electrode,whereby lithium is diffused into the niobium oxide layer 110 so that theniobium oxide layer 110 is doped with lithium; consequently, Li_(x)Nb₂O₅(x>0) is formed.

Note that the niobium oxide layer 110 may be doped with all the lithiumin the lithium layer 116 by the predoping or part of the lithium layer116 may be left. The part of the lithium layer 116 left after thepredoping can be used to compensate lithium lost by irreversiblecapacity due to charge and discharge of the power storage element.

Although the insulating layer 102 is provided in this embodiment, astructure without the insulating layer 102 may be employed. For example,in the case where an insulating material is used for the substrate 100,the insulating layer 102 can be omitted.

There is no particular limitation on the substrate 100 as long as it hasheat resistance enough to withstand at least heat treatment performedlater. For example, a glass substrate, a ceramic substrate, a quartzsubstrate, or a sapphire substrate may be used as the substrate 100.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like; a compound semiconductor substrate made of GaN orthe like; a silicon-on-insulator (SOI) substrate; or the like may beused as the substrate 100. Still alternatively, any of these substratesfurther provided with a semiconductor element such as a transistor maybe used as the substrate 100.

Further alternatively, a flexible substrate may be used as the substrate100. Note that as a method for forming a power storage element over aflexible substrate, there is a method in which after a power storageelement is formed over a non-flexible substrate, the power storageelement is separated from the non-flexible substrate and transferred toa flexible substrate. In that case, a separation layer is preferablyprovided between the non-flexible substrate and the power storageelement.

The insulating layer 102 may be formed to have a single-layer or layeredstructure using at least one of the following materials: silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, aluminum nitride, hafnium oxide, zirconium oxide, yttrium oxide,gallium oxide, lanthanum oxide, cesium oxide, tantalum oxide, andmagnesium oxide.

The vanadium layer 104 and the niobium layer 106 can be fanned by asputtering method, a vacuum deposition method, or the like. When asputtering method is used, not an RF power supply but a DC power supplyis used for deposition because vanadium has high conductivity. Asputtering method using a DC power supply is preferable because thedeposition rate is high and thus cycle time is short. The thickness maybe, for example, greater than or equal to 100 nm and less than or equalto 10 μm. Alternatively, the vanadium layer 104 and the niobium layer106 may be formed by nanoimprint lithography.

The vanadium layer or the niobium layer may be foamed over a selectivelyformed current collector layer. For example, a layered structure may beemployed in which the vanadium layer or the niobium layer is formed overa current collector selectively formed using titanium, aluminum, copper,or the like. Alternatively, a layered structure including more than twolayers may be employed.

The distance between the vanadium layer 104 and the niobium layer 106may be, for example, greater than or equal to 10 nm and is preferablygreater than or equal to 100 nm for ease of manufacture. Such a distanceis provided to prevent a short circuit. However, a wide distance betweenthe vanadium layer 104 serving as a positive electrode current collectorand the niobium layer 106 serving as a negative electrode currentcollector decreases the conductivity of carrier ions. Thus, the distancebetween the vanadium layer 104 and the niobium layer 106 isappropriately adjusted in accordance with the ionic conductivity of asolid electrolyte used in a secondary battery.

Note that either the vanadium layer 104 or the niobium layer 106 may beformed first.

In this embodiment, the vanadium layer is used as the positive electrodecurrent collector layer; however, the present invention is not limitedthereto. A layer containing manganese may be used as a positiveelectrode current collector layer.

Further, in this embodiment, the niobium layer is used as the negativeelectrode current collector layer; however, the present invention is notlimited thereto. A layer containing copper, cobalt, nickel, iron,tungsten, molybdenum, or tantalum may be used as a negative electrodecurrent collector layer.

The vanadium oxide layer 108 can be formed by performing oxidationtreatment on a surface of the vanadium layer 104. Similarly, the niobiumoxide layer 110 can be formed by performing oxidation treatment on asurface of the niobium layer 106. Note that oxidation treatment forformation of the vanadium oxide layer 108 and oxidation treatment forformation of the niobium oxide layer 110 can be performed at one time.Thus, the vanadium oxide layer 108 and the niobium oxide layer 110 canbe formed simultaneously, so that the process can be simplified.

Also in the case of using a positive electrode current collector layerformed using any of the above elements other than vanadium and anegative electrode current collector layer formed using any of the aboveelements other than niobium, oxidation treatment is performed, wherebylayers serving as positive electrode active material layers and negativeelectrode active material layers can be formed.

The vanadium oxide layer 108 and the niobium oxide layer 110 function asa positive electrode active material and a negative electrode activematerial, respectively. Thus, the thickness of the vanadium oxide layer108 and the niobium oxide layer 110 may be adjusted in accordance withnecessary battery capacity. Note that complete oxidation of the vanadiumlayer 104 and the niobium layer 106 results in the loss of the functionsof the positive electrode current collector and the negative electrodecurrent collector. Thus, for example, in the case of performingoxidation treatment on the vanadium layer 104 to form the vanadium oxidelayer 108, treatment conditions are adjusted so that 10% or more of thevanadium layer 104 which has not been subjected to treatment is left.

For the solid electrolyte layer 112, an inorganic solid electrolytewhich can be formed by a sputtering method, an evaporation method, or achemical vapor deposition method (specifically a metal-organic chemicalvapor deposition method) may be used. Examples of the inorganic solidelectrolyte are a sulfide-based solid electrolyte and an oxide-basedsolid electrolyte.

Examples of the sulfide-based solid electrolyte are compounds containinglithium and sulfur such as Li₂S—SiS₂—Li₃PO₄, Li₂S-P₂S₅, Li₂S—SiS₂—Ga₂S₃,LiI—Li₂S—P₂S₅, LiI—Li₂S—B₂S₃, LiI—Li₂S—SiS₂, Li₃PO₄—Li₂S—SiS₂, andLi₄SiO₄—Li₂S—SiS₂.

Examples of the oxide-based solid electrolyte are compounds containinglithium and sulfur, compounds containing lithium and oxygen, and lithiumoxides such as LiPON, Li₂O, Li₂CO₃, Li₂MoO₄, Li₃PO₄, Li₃VO₄, Li₄SiO₄,LLT(La_(2/3-x)Li_(3x)TiO₃), and LLZ(Li₇La₃Zr₂O₁₂).

Alternatively, a polymer solid electrolyte such as poly(ethylene oxide)(PEO) formed by a coating method or the like may be used. Stillalternatively, a composite solid electrolyte containing any of the aboveinorganic solid electrolytes and a polymer solid electrolyte may beused.

The lithium layer 116 may be formed by a sputtering method, a vacuumdeposition method, or the like. The thickness of the lithium layer 116is appropriately determined depending on the amount needed forpredoping. Note that the lithium layer 116 is formed so as to overlapwith at least one of the vanadium oxide layer 108 and the niobium oxidelayer 110 and may be formed over the entire surface over which thelithium layer 116 is to be formed. Although the lithium layer 116 isformed so as to overlap with the niobium oxide layer 110 with the solidelectrolyte layer 112 interposed therebetween in FIG. 1C, the lithiumlayer 116 may be in direct contact with the niobium oxide layer 110.

The sealing layer 114 is also referred to as a capping layer. Thesealing layer 114 is formed to cover the solid electrolyte layer 112,the vanadium oxide layer 108, and the niobium oxide layer 110. Thesealing layer 114 formed in this manner can prevent the power storageelement 200 from being exposed to the air. For the sealing layer 114, aninsulating material such as a resin (e.g., epoxy resin), glass, anamorphous compound, or ceramics can be used, for example. Further, alayer containing calcium fluoride or the like may be provided as a waterabsorption layer in an epoxy resin layer. The sealing layer 114 can beformed by a spin coating method, an ink-jet method, or the like.

(Formation Method of Power Storage Element)

Next, a formation method of the power storage element illustrated inFIG. 1C will be described with reference to FIGS. 2A to 2D.

First, the insulating layer 102 is formed over the substrate 100. Theinsulating layer 102 can be formed by a sputtering method, a CVD method,an evaporation method, or the like. In this embodiment, silicon oxide ispreferably deposited to a thickness of approximately 100 nm.

Then, the vanadium layer 104 is formed over the insulating layer 102.The vanadium layer 104 can be formed by a sputtering method, forexample. A sputtering method using a DC power supply is preferablyemployed for formation of the vanadium layer 104 because vanadium hashigh conductivity. The thickness of the vanadium layer 104 is greaterthan or equal to 100 nm and less than or equal to 10 μm, preferablygreater than or equal to 1 μm and less than or equal to 3 μm.

Processing is performed so that the vanadium layer 104 has a desiredshape to function as the positive electrode current collector, afterdeposition is performed by a deposition method. Alternatively, forexample, the vanadium layer 104 is formed by a sputtering method using ametal mask or the like, whereby the vanadium layer 104 can be providedto have a desired shape without a step such as processing.

Alternatively, the vanadium layer 104 may be formed by nanoimprintlithography. In nanoimprint lithography, first, the surface of aplate-shaped mold formed of a resin or the like is processed to have adesired shape; then, the mold processed to have the desired shape isbrought into contact with (stamped on) a board over which a material tobe deposited (e.g., a vanadium paste) is evenly applied, in order thatthe material is selectively transferred to the mold, and the selectivelytransferred material is brought into contact with a surface over whichthe material is to be deposited, whereby the material can be selectivelydeposited.

Still alternatively, a vanadium layer may be processed byphotolithography after being formed by a sputtering method. For example,a photoresist is subjected to light exposure to form a mask over theformed vanadium layer and etching is performed using hydrofluoric acid,so that the processed vanadium layer 104 can be formed.

Then, the niobium layer 106 is formed over the insulating layer 102. Theniobium layer 106 can be formed by a sputtering method, for example. Asputtering method using a DC power supply is preferably employed forformation of the niobium layer 106 because niobium has highconductivity. The thickness of the niobium layer 106 is greater than orequal to 100 nm and less than or equal to 10 nm, preferably greater thanor equal to 1 μm and less than or equal to 3 μm.

Processing is performed so that the niobium layer 106 has a desiredshape to function as the negative electrode current collector, afterdeposition is performed by a deposition method. Alternatively, forexample, the niobium layer 106 is formed by a sputtering method using ametal mask or the like, whereby the niobium layer 106 can be provided tohave a desired shape without a step such as processing.

Alternatively, the niobium layer 106 may be formed by nanoimprintlithography.

Still alternatively, a niobium layer may be processed byphotolithography after being formed by a sputtering method. For example,a photoresist is subjected to light exposure to form a mask over theformed niobium layer and etching is performed using an alkaline solutionsuch as a potassium hydroxide aqueous solution, so that the processedniobium layer 106 can be formed.

Through the above steps, the vanadium layer 104 functioning as thepositive electrode current collector and the niobium layer 106functioning as the negative electrode current collector can be formed.

Then, the vanadium layer 104 and the niobium layer 106, which have beenformed, are subjected to oxidation treatment such as oxygen plasmatreatment, radical oxidation treatment, anodic oxidation treatment, orthermal oxidation treatment (see FIG. 2A).

By the oxidation treatment, the surfaces of the vanadium layer 104 andthe niobium layer 106 are oxidized, so that the vanadium oxide layer 108and the niobium oxide layer 110 are formed (see FIG. 2B). Anodicoxidation treatment enables formation of a thick oxide film and thus issuitable for forming the vanadium oxide layer 108 and the niobium oxidelayer 110 so that they are thick.

Then, the solid electrolyte layer 112 is formed in contact with thevanadium oxide layer 108 and the niobium oxide layer 110. For the solidelectrolyte layer 112, LiPON may be used, for example. LiPON can bedeposited by a sputtering method; specifically, a Li₃PO₄ target and areactive sputtering method using a reaction gas containing a nitrogengas can be used. The thickness of the solid electrolyte layer 112 isgreater than or equal to 100 nm and less than or equal to 10 μm.Further, when deposition is performed using a metal mask, the solidelectrolyte layer 112 having a desired shape can be formed without astep such as processing.

Alternatively, nanoimprint lithography or photolithography may beemployed for forming the solid electrolyte layer 112.

Then, the lithium layer 116 is formed so as to overlap with the niobiumoxide layer 110 with the solid electrolyte layer 112 interposedtherebetween (see FIG. 2C). The lithium layer 116 may be formed by anevaporation method, a sputtering method, or the like. The thickness ofthe lithium layer 116 is appropriately determined depending on theamount needed for predoping. Note that the lithium layer 116 is formedso as to overlap with at least one of the vanadium oxide layer 108 andthe niobium oxide layer 110 and may be formed over the entire surfaceover which the lithium layer 116 is to be formed. Although the lithiumlayer 116 is formed so as to overlap with the niobium oxide layer 110 inFIG. 1C, the lithium layer 116 may be in direct contact with the niobiumoxide layer 110. The lithium layer 116 in direct contact with theniobium oxide layer 110 allows the niobium oxide layer 110 to bepredoped with lithium without a process such as application of anelectric field. Note that the same applies to predoping the vanadiumoxide layer 108.

Then, the sealing layer 114 is formed to cover the lithium layer 116,the solid electrolyte layer 112, the vanadium oxide layer 108, and theniobium oxide layer 110 (see FIG. 2D). For the sealing layer 114, anepoxy resin may be used, for example. The sealing layer 114 is providedto prevent the power storage element 200 from being exposed to externalair, leading to minimization of deterioration of the power storageelement 200.

Through the above steps, the power storage element 200 in FIG. 1C can beformed.

As described in one embodiment of the present invention, oxidationtreatment is performed on the positive electrode current collector layerand the negative electrode current collector layer, whereby the positiveelectrode active material layer and the negative electrode activematerial layer can be formed on the surface of the positive electrodecurrent collector layer and the surface of the negative electrodecurrent collector layer, respectively. Thus, the manufacturing processcan be simplified, leading to a reduction in manufacturing cost.

According to one embodiment of the present invention, a power storageelement which includes a positive electrode and a negative electrodeprovided so as to be level with each other and which can be formedthrough a simple process and a formation method of the power storageelement can be provided.

(Embodiment 2)

A power storage device in this embodiment is different from the powerstorage device 900 in Embodiment 1 in that the plurality of powerstorage elements 200 are electrically connected through wirings.Specifically, in this embodiment, the plurality of power storageelements 201 arranged so as to be level with each other in a powerstorage device 901 are electrically connected through wirings 218,whereby the power storage elements 201 are arranged in series or inparallel.

In this embodiment, as in Embodiment 1, descriptions will be given of apower storage element and a power storage device in each of which avanadium layer is used as a positive electrode current collector layer;a vanadium oxide layer is used as a positive electrode active materiallayer; a niobium layer is used as a negative electrode current collectorlayer; and a niobium oxide layer is used as a negative electrode activematerial layer.

In this embodiment, a means for switching between series connection andparallel connection of the power storage elements (also referred to as aswitch) may be provided. A semiconductor device including a transistorand the like may be used as the switching means, for example.

FIG. 3A is a top view of the power storage device 901. As illustrated inFIG. 3A, the plurality of power storage elements 201 are arranged andelectrically connected through the wirings 218. Although notparticularly illustrated, a switch for electrical connection can beprovided between the power storage elements 201. FIG. 3B is a top viewof the power storage element 201 and FIG. 3C is a cross-sectional viewalong dashed dotted line A-B in FIG. 3B. Note that the differencebetween the power storage element 201 and the power storage element 200in Embodiment 1 is presence or absence of the wirings 218 connected tothe vanadium layer 204 and the niobium layer 206.

The wiring 218 is preferably formed using a material with highconductivity. For the wiring 218, a metal film containing an elementselected from gold (Au), platinum (Pt), aluminum (Al), chromium (Cr),copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), and tungsten(W) or a metal nitride film containing any of the above elements as itscomponent (e.g., a titanium nitride film, a molybdenum nitride film, ora tungsten nitride film) can be used.

The wiring 218 can be formed by a sputtering method, a CVD method, anevaporation method, or the like. Further, a metal mask can be used information of the wiring 218 by any of the methods, whereby a desiredshape can be formed without processing. Alternatively, nanoimprintlithography may be employed, in which case the wirings 218 each having adesired shape can be formed without a step for processing as in the caseof using a metal mask.

As described above, the plurality of power storage elements 201 areelectrically connected through the wirings 218 and switches with whichconnection of the power storage elements 201 is switched are provided.The switch can be formed using a semiconductor device including atransistor and the like. Although the semiconductor device can beprovided so as to be level with the power storage element 201, a spacefor the power storage element 201 is reduced by the semiconductordevice. Thus, the semiconductor device and the power storage element 201are preferably formed in different layers; for example, the powerstorage element 201 is formed in a layer over the semiconductor device,after the semiconductor device is formed over a substrate.

Next, a manufacturing method of a structure where a semiconductor device(here, a transistor 501) and the power storage element 201 are stackedwill be described with reference to FIGS. 4A to 4D and FIGS. 5A to 5D.Note that the transistor 501 described below is only an example and isnot limited thereto.

<Formation Method of Transistor 501>

First, the substrate 100 is prepared. There is no particular limitationon the substrate 100 as long as it has heat resistance enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 100. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate of silicon, silicon carbide, or the like, a compoundsemiconductor substrate of silicon germanium or the like, an SOI(silicon on insulator) substrate, or the like can be used.

Still alternatively, a flexible substrate may be used as the substrate100. Note that as a method for forming a transistor over a flexiblesubstrate, there is a method in which after a transistor is formed overa non-flexible substrate, the transistor is separated from thenon-flexible substrate and transferred to the substrate 100 which isflexible. In that case, a separation layer is preferably providedbetween the non-flexible substrate and the transistor.

Then, the base insulating film 502 is formed. The base insulating film502 may be formed to have a single-layer or layered structure using oneor more of the following materials: aluminum oxide, magnesium oxide,silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanumoxide, neodymium oxide, hafnium oxide, and tantalum oxide.

Next, a semiconductor film to be the semiconductor layer 504 is formedusing a Group 14 element such as silicon or germanium, or a metal oxidesuch as In—Ga—Zn-based oxide. The semiconductor film may be formed by asputtering method, a CVD method, an MBE method, an ALD method, or a PLDmethod.

Next, the semiconductor film to be the semiconductor layer 504 isprocessed into an island shape. For example, etching treatment isperformed using a resist mask that is formed by a photolithographymethod, whereby the semiconductor layer 504 having a desired shape isobtained.

Then, a gate insulating film 506 is formed (see FIG. 4A). The gateinsulating film 506 may be formed to have a single-layer or layeredstructure using one or more of the following materials: aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Thegate insulating film 506 may be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, or a PLD method.

Then, a conductive film to be a gate electrode 508 is formed. Theconductive film may be formed to have a single-layer or layeredstructure using a simple substance, a nitride, an oxide, or an alloycontaining an element selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo,Ag, Ta, and W. The conductive film may be formed by a sputtering method,a CVD method, an MBE method, an ALD method, or a PLD method.

The conductive film is processed to form the gate electrode 508, and animpurity which generates a carrier in the semiconductor layer 504 isadded using the gate electrode 508 as a mask (see FIG. 4B).

When an impurity is added to the semiconductor layer 504, a channelregion 503 to which the impurity is not added and low resistance regions505 to which the impurity is added are formed (see FIG. 4C). As theimpurity, phosphorus, boron, or the like may be added in the case wherethe semiconductor layer is formed using silicon or a metal oxide such asIn-Ga-Zn-based oxide.

Then, an interlayer insulating film 510 is formed. The interlayerinsulating film 510 may be formed using a method and a material that aresimilar to those of the base insulating film 502.

Then, the interlayer insulating film 510 is processed, and the wiring218 is formed in contact with the semiconductor layer 504. The wiring218 can be formed by processing a conductive film to be the wiring 218(see FIG. 4D). Although the structure where the wiring 218 is connectedto the semiconductor layer 504 of the transistor 501 is described inthis embodiment, the structure of the transistor 501 is not limitedthereto. For example, the gate electrode 508 may be connected to thewiring 218.

Through the above steps, the transistor 501 can be formed.

<Formation Method of Power Storage Element 201>

Next, a method for forming the power storage element 201 above thetransistor 501 formed through the above steps will be described below.

First, the vanadium layer 204 is formed over the interlayer insulatingfilm 510. The vanadium layer 204 can be formed by a sputtering method,for example. A sputtering method using a DC power supply is preferablyemployed for formation of the vanadium layer 204 because vanadium hashigh conductivity. The thickness of the vanadium layer 204 is greaterthan or equal to 100 nm and less than or equal to 10 μm, or greater thanor equal to 1 μm and less than or equal to 3 μm.

The vanadium layer 204 can be prepared by employing materials andmethods similar to those employed for the vanadium layer 104 describedin Embodiment 1.

Then, the niobium layer 206 is formed over the interlayer insulatingfilm 510. The niobium layer 206 can be formed by a sputtering method,for example. A sputtering method using a DC power supply is preferablyemployed for formation of the niobium layer 206 because niobium has highconductivity. The thickness of the niobium layer 206 is greater than orequal to 100 nm and less than or equal to 10 μm, or greater than orequal to 1 μm and less than or equal to 3 μm.

The niobium layer 206 can be prepared by employing materials and methodssimilar to those employed for the niobium layer 106 described inEmbodiment 1.

Through the above steps, the vanadium layer 204 serving as the positiveelectrode current collector and the niobium layer 206 serving as thenegative electrode current collector can be formed.

Then, the vanadium layer 204 and the niobium layer 206, which have beenformed, are subjected to oxidation treatment such as oxygen plasmatreatment, radical oxidation treatment, anodic oxidation treatment, orthermal oxidation treatment (see FIG. 5A).

By the oxidation treatment, the surfaces of the vanadium layer 204 andthe niobium layer 206 are oxidized, whereby the vanadium oxide layer 208and the niobium oxide layer 210 are formed (see FIG. 5B).

Then, the solid electrolyte layer 112 is formed in contact with thevanadium oxide layer 208 and the niobium oxide layer 210. The solidelectrolyte layer 112 can be prepared by employing a material and amethod at a thickness similar to those employed for the solidelectrolyte layer 112 described in Embodiment 1.

Then, the lithium layer 116 is formed so as to overlap with the niobiumoxide layer 210 with the solid electrolyte layer 112 interposedtherebetween (see FIG. 5C). The lithium layer 116 can be prepared byemploying methods similar to those employed for the lithium layer 116described in Embodiment 1.

Then, the sealing layer 114 is formed to cover the lithium layer 116,the solid electrolyte layer 112, the vanadium oxide layer 208, and theniobium oxide layer 210 (see FIG. 5D). The sealing layer 114 can beprepared by employing materials and methods similar to those employedfor the sealing layer 114 described in Embodiment 1.

Although the vanadium oxide layer 208 and the niobium oxide layer 210are formed by performing oxidation treatment on the vanadium layer 204and the niobium layer 206 in FIG. 5B, a surface of the wiring 218 mayalso be oxidized by the oxidation treatment. For example, in the casewhere aluminum or an aluminum alloy is used for the wiring 218, asurface of the aluminum is oxidized by oxidation treatment, so thataluminum oxide in the passive state is formed. The use of a metal oxidefilm 220 formed on the surface of the wiring 218 in such a manner (seeFIG. 6) allows formation of a highly reliable power storage device whichhas resistance to the deterioration of the wiring due to moisture or thelike from external air.

Through the above steps, the power storage element 201 can be formedabove the transistor 501.

Next, FIG. 7A is a block diagram illustrating connection in the powerstorage device 901 where the plurality of power storage elements 201 areelectrically connected through the wirings 218 in FIG. 3A. Note that apower storage element array 300 includes the plurality of power storageelements 201.

As illustrated in FIG. 7B, in the power storage element array 300, theplurality of power storage elements 201 are connected to each other, anda plurality of switches 400 are provided between the adjacent powerstorage elements 201.

With the switches 400, the connection of the wirings 218 can beswitched, whereby switching between series connection and parallelconnection of the power storage elements 201 in the power storageelement array 300 can be performed. Additionally, series-parallelconnection in which series connection and parallel connection arecombined can be employed.

For example, by switching the connection of the switches 400 asillustrated in FIG. 8A, the power storage elements 201 are connected inseries.

Further, by switching the connection of the switches 400 as illustratedin FIG. 8B, the power storage elements 201 are connected in parallel.

As described above, by switching the connection of the switches 400,switching between series connection and parallel connection of theplurality of power storage elements 201 can be easily performed. Thus,for example, in the case where a high electromotive force is needed, thepower storage elements 201 are connected in series, whereby theelectromotive force can be increased in accordance with the number ofconnected elements. When the power storage elements 201 are connected inparallel, the power storage elements 201 can be charged all at once,leading to a reduction in charging time.

According to this embodiment, various potentials according to the numberof serially connected power storage elements can be supplied evensimultaneously. For example, a circuit including a plurality of elements(e.g., CPU, DRAM, and SRAM), such as LSI, needs to be provided with aplurality of power supply circuits because the plurality of elementsneed different potentials. In contrast, the power storage device of thisembodiment enables various potentials to be supplied simultaneously, sothat such a plurality of power supply circuits are unnecessary.Therefore, reductions in scale and cost of a circuit can be achieved.

As described in one embodiment of the present invention, oxidationtreatment is performed on the positive electrode current collector layerand the negative electrode current collector layer, whereby the positiveelectrode active material layer and the negative electrode activematerial layer can be formed on the surface of the positive electrodecurrent collector layer and the surface of the negative electrodecurrent collector layer, respectively, at the same time. Thus, themanufacturing process can be simplified, leading to a reduction inmanufacturing cost.

According to one embodiment of the present invention, a power storageelement which includes a positive electrode and a negative electrodeprovided so as to be level with each other and which can be formedthrough a simple process, a formation method of the power storageelement, and a power storage device can be provided.

Note that circuit diagrams in FIG. 7B and FIGS. 8A and 8B are justexamples; a circuit where the arrangement of the power storage elements201 and the switches 400 is changed as appropriate may be used.

(Embodiment 3)

In this embodiment, application examples of any of the power storagedevices described in Embodiments 1 and 2 will be described withreference to FIG. 9.

Specific examples of electrical devices each utilizing the power storagedevice of one embodiment of the present invention are as follows:display devices, lighting devices, desktop personal computers and laptoppersonal computers, image reproduction devices which reproduce stillimages and moving images stored in recording media such as digitalversatile discs (DVDs), mobile phones, portable game machines, portableinformation terminals, e-book readers, video cameras, digital stillcameras, high-frequency heating appliances such as microwave ovens,electric rice cookers, electric washing machines, air-conditioningsystems such as air conditioners, electric refrigerators, electricfreezers, and electric refrigerator-freezers, freezers for preservingDNA, and dialyzers. In addition, moving objects driven by electricmotors using power from power storage devices are also included in thecategory of electrical devices. Examples of the moving objects includeelectric vehicles, hybrid vehicles each including both aninternal-combustion engine and an electric motor, and motorized bicyclesincluding motor-assisted bicycles.

In the electrical devices, the power storage device of one embodiment ofthe present invention can be used as a power storage device forsupplying enough power for almost the whole power consumption (referredto as a main power supply). Alternatively, in the electrical devices,the power storage device of one embodiment of the present invention canbe used as a power storage device to supply power to the electricaldevices when supply of power from a main power supply or a commercialpower supply is stopped (such a power storage device is referred to asan uninterruptible power supply). Still alternatively, in the electricaldevices, the power storage device of one embodiment of the presentinvention can be used as a power storage device for supplying power tothe electrical devices at the same time as the power supply from a mainpower supply or a commercial power supply (such a power storage deviceis referred to as an auxiliary power supply).

FIG. 9 illustrates specific structures of the electrical devices. InFIG. 9, a display device 5000 is an example of an electrical deviceincluding a power storage device 5004 of one embodiment of the presentinvention. Specifically, the display device 5000 corresponds to adisplay device for TV broadcast reception and includes a housing 5001, adisplay portion 5002, speaker portions 5003, and the power storagedevice 5004. The power storage device 5004 of one embodiment of thepresent invention is provided in the housing 5001. The display device5000 can receive power from a commercial power supply. Alternatively,the display device 5000 can use power stored in the power storage device5004. Thus, the display device 5000 can be operated with the use of thepower storage device 5004 of one embodiment of the present invention asan uninterruptible power supply even when power cannot be supplied froma commercial power supply due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 5002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 9, an installation lighting device 5100 is an example of anelectrical appliance including a power storage device 5103 of oneembodiment of the present invention. Specifically, the lighting device5100 includes a housing 5101, a light source 5102, and a power storagedevice 5103. Although FIG. 9 illustrates the case where the powerstorage device 5103 is provided in a ceiling 5104 on which the housing5101 and the light source 5102 are installed, the power storage device5103 may be provided in the housing 5101. The lighting device 5100 canreceive power from a commercial power supply. Alternatively, thelighting device 5100 can use power stored in the power storage device5103. Thus, the lighting device 5100 can be operated with the use of thepower storage device 5103 of one embodiment of the present invention asan uninterruptible power supply even when power cannot be supplied froma commercial power supply due to power failure or the like.

Note that although the installation lighting device 5100 provided in theceiling 5104 is illustrated in FIG. 9 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 5105, afloor 5106, a window 5107, or the like other than the ceiling 5104.Alternatively, the power storage device can be used in a tabletoplighting device or the like.

As the light source 5102, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and light-emittingelements such as an LED and an organic EL element are given as examplesof the artificial light source.

In FIG. 9, an air conditioner including an indoor unit 5200 and anoutdoor unit 5204 is an example of an electrical appliance including apower storage device 5203 of one embodiment of the invention.Specifically, the indoor unit 5200 includes a housing 5201, an airoutlet 5202, and a power storage device 5203. Although FIG. 9illustrates the case where the power storage device 5203 is provided inthe indoor unit 5200, the power storage device 5203 may be provided inthe outdoor unit 5204. Alternatively, the power storage devices 5203 maybe provided in both the indoor unit 5200 and the outdoor unit 5204. Theair conditioner can receive power from a commercial power supply.Alternatively, the air conditioner can use power stored in the powerstorage device 5203. Particularly in the case where the power storagedevices 5203 are provided in both the indoor unit 5200 and the outdoorunit 5204, the air conditioner can be operated with the use of the powerstorage device 5203 of one embodiment of the present invention as anuninterruptible power supply even when power cannot be supplied from acommercial power supply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 9 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 9, an electric refrigerator-freezer 5300 is an example of anelectrical appliance including a power storage device 5304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 5300 includes a housing 5301, a door for arefrigerator 5302, a door for a freezer 5303, and the power storagedevice 5304. The power storage device 5304 is provided in the housing5301 in FIG. 9. The electric refrigerator-freezer 5300 can receive powerfrom a commercial power supply. Alternatively, the electricrefrigerator-freezer 5300 can use power stored in the power storagedevice 5304. Thus, the electric refrigerator-freezer 5300 can beoperated with the use of the power storage device 5304 of one embodimentof the present invention as an uninterruptible power supply even whenpower cannot be supplied from a commercial power supply due to powerfailure or the like.

Note that among the electrical devices described above, a high-frequencyheating apparatus such as a microwave oven and an electrical device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of anelectrical appliance can be prevented by using the power storage deviceof one embodiment of the present invention as an auxiliary power supplyfor supplying power which cannot be supplied enough by a commercialpower supply.

In addition, in a time period when electrical devices are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power supply source (such a proportion referred to as a usagerate of power) is low, power can be stored in the power storage device,whereby the usage rate of power can be reduced in a time period when theelectrical devices are used. For example, in the case of the electricrefrigerator-freezer 5300, power can be stored in the power storagedevice 5304 in night time when the temperature is low and the door for arefrigerator 5302 and the door for a freezer 5303 are not often openedor closed. On the other hand, in daytime when the temperature is highand the door for a refrigerator 5302 and the door for a freezer 5303 arefrequently opened and closed, the power storage device 5304 is used asan auxiliary power supply; thus, the usage rate of power in daytime canbe reduced.

Next, a portable information terminal which is an example of electricaldevices will be described with reference to FIGS. 10A to 10C.

FIGS. 10A and 10B illustrate a tablet terminal which can be folded. FIG.10A illustrates the tablet terminal in the state of being unfolded. Thetablet terminal includes a housing 9630, a display portion 9631 a, adisplay portion 9631 b, a display-mode switching button 9034, a powerbutton 9035, a power-saving-mode switching button 9036, a fastener 9033,and an operation button 9038.

A touch panel area 9632 a can be provided in part of the display portion9631 a, in which area, data can be input by touching displayed operationkeys 9638. Note that half of the display portion 9631 a has only adisplay function and the other half has a touch panel function. However,the structure of the display portion 9631 a is not limited to this, andall the area of the display portion 9631 a may have a touch panelfunction. For example, a keyboard can be displayed on the whole displayportion 9631 a to be used as a touch panel, and the display portion 9631b can be used as a display screen.

A touch panel area 9632 b can be provided in part of the display portion9631 b like in the display portion 9631 a. When a keyboard displayswitching button 9639 displayed on the touch panel is touched with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 9631 b.

The touch panel area 9632 a and the touch panel area 9632 b can becontrolled by touch input at the same time.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power-saving-mode switching button 9036allows optimizing the display luminance in accordance with the amount ofexternal light in use which is detected by an optical sensorincorporated in the tablet terminal. In addition to the optical sensor,other detecting devices such as sensors for determining inclination,such as a gyroscope or an acceleration sensor, may be incorporated inthe tablet terminal.

Although the display area of the display portion 9631 a is the same asthat of the display portion 9631 b in FIG. 10A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 9631 a may be different from that of the displayportion 9631 b, and further, the display quality of the display portion9631 a may be different from that of the display portion 9631 b. Forexample, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

FIG. 10B illustrates the tablet terminal in the state of being closed.The tablet terminal includes the housing 9630, a solar cell 9633, acharge/discharge control circuit 9634, a battery 9635, and a DC-DCconverter 9636. FIG. 10B illustrates an example where thecharge/discharge control circuit 9634 includes the battery 9635 and theDC-DC converter 9636. The power storage device described in the aboveembodiment is used as the battery 9635.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen the tablet terminal is not in use. Thus, the display portions 9631a and 9631 b can be protected, which permits the tablet terminal to havehigh durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 10A and 10B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 9633, which is attached on a surface of the tabletterminal, can supply electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thus thebattery 9635 can be charged efficiently. The use of the power storagedevice of one embodiment of the present invention as the battery 9635has advantages such as a reduction in size.

The structure and operation of the charge/discharge control circuit 9634illustrated in FIG. 10B will be described with reference to a blockdiagram of FIG. 10C. FIG. 10C illustrates the solar cell 9633, thebattery 9635, the DC-DC converter 9636, a converter 9637, switches SW1to SW3, and the display portion 9631. The battery 9635, the DC-DCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 10B.

First, an example of operation in the case where electric power isgenerated by the solar cell 9633 using external light will be described.The voltage of electric power generated by the solar cell 9633 is raisedor lowered by the DC-DC converter 9636 so that the electric power has avoltage for charging the battery 9635. When the display portion 9631 isoperated with the electric power from the solar cell 9633, the switchSW1 is turned on and the voltage of the electric power is raised orlowered by the converter 9637 to a voltage needed for operating thedisplay portion 9631. In addition, when display on the display portion9631 is not performed, the switch SW1 is turned off and the switch SW2is turned on so that the battery 9635 may be charged.

Although the solar cell 9633 is described as an example of a powergeneration means, there is no particular limitation on the powergeneration means, and the battery 9635 may be charged with any of theother means such as a piezoelectric element or a thermoelectricconversion element (Peltier element). For example, the battery 9635 maybe charged with a non-contact power transmission module capable ofperforming charging by transmitting and receiving electric powerwirelessly (without contact), or any of the other charge means used incombination.

It is needless to say that one embodiment of the present invention isnot limited to the electrical device illustrated in FIGS. 10A to 10C aslong as the electrical device is equipped with the power storage devicedescribed in the above embodiment.

This embodiment can be implemented in combination with any of the aboveembodiments as appropriate.

This application is based on Japanese Patent Application serial no.2012-069536 filed with the Japan Patent Office on Mar. 26, 2012, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A power storage device comprising: a plurality ofpower storage elements; and a wiring electrically connected to theplurality of power storage elements, wherein each of the plurality ofpower storage elements comprises: a pair of electrodes in a same plane;a first layer over and in contact with the pair of electrodes; and asecond layer over the first layer, wherein the first layer comprises asolid electrolyte, and wherein the second layer comprises a carriersupplying material.
 2. The power storage device according to claim 1,wherein the plurality of power storage elements are connected in seriesor in parallel.
 3. The power storage device according to claim 2,further comprising a switch for switching between series connection andparallel connection.
 4. The power storage device according to claim 1,wherein one of the pair of electrodes comprises a first metal and anoxide of the first metal, and wherein the other of the pair ofelectrodes comprises a second metal and an oxide of the second metal. 5.The power storage device according to claim 4, wherein the first metalis vanadium or manganese, and wherein the second metal is any one ofniobium, copper, cobalt, nickel, iron, tungsten, molybdenum, andtantalum.
 6. The power storage device according to claim 4, wherein theoxide of the first metal covers upper and side surfaces of the firstmetal, and wherein the oxide of the second metal covers upper and sidesurfaces of the second metal.
 7. The power storage device according toclaim 1, wherein the solid electrolyte is any one of a sulfide-basedsolid electrolyte containing lithium, an oxide-based solid electrolytecontaining lithium, and a polymer solid electrolyte.
 8. The powerstorage device according to claim 1, wherein the carrier supplyingmaterial is any one of lithium, sodium, potassium, calcium, strontium,barium, beryllium, and magnesium.
 9. The power storage device accordingto claim 1, wherein each of the plurality of power storage elementsfurther comprises a sealing layer over the pair of electrodes, the firstlayer, and the second layer.
 10. A power storage device comprising: atransistor; a plurality of power storage elements; and a wiringelectrically connected to the transistor and the plurality of powerstorage elements, wherein each of the plurality of power storageelements comprises: a pair of electrodes in a same plane; a first layerover and in contact with the pair of electrodes; and a second layer overthe first layer, wherein the first layer comprises a solid electrolyte,and wherein the second layer comprises a carrier supplying material. 11.The power storage device according to claim 10, wherein the plurality ofpower storage elements are connected in series or in parallel.
 12. Thepower storage device according to claim 10, wherein the transistor isconfigured to switch between series connection and parallel connection.13. The power storage device according to claim 10, wherein one of thepair of electrodes comprises a first metal and an oxide of the firstmetal, and wherein the other of the pair of electrodes comprises asecond metal and an oxide of the second metal.
 14. The power storagedevice according to claim 13, wherein the first metal is vanadium ormanganese, and wherein the second metal is any one of niobium, copper,cobalt, nickel, iron, tungsten, molybdenum, and tantalum.
 15. The powerstorage device according to claim 13, wherein the oxide of the firstmetal covers upper and side surfaces of the first metal, and wherein theoxide of the second metal covers upper and side surfaces of the secondmetal.
 16. The power storage device according to claim 10, wherein thesolid electrolyte is any one of a sulfide-based solid electrolytecontaining lithium, an oxide-based solid electrolyte containing lithium,and a polymer solid electrolyte.
 17. The power storage device accordingto claim 10, wherein the carrier supplying material is any one oflithium, sodium, potassium, calcium, strontium, barium, beryllium, andmagnesium.
 18. The power storage device according to claim 10, whereineach of the plurality of power storage elements further comprises asealing layer over the pair of electrodes, the first layer, and thesecond layer.